The manufacture of semiconductors is one of the most competitive fields in industry today, due in part to the commodity nature of the business. To produce the most cost competitive product possible, throughput must be maximized. The higher quantity of chips a company can produce, the lower the per unit cost for the product. Speed and accuracy are often at odds, and finding the most profitable balance can be of utmost importance in maintaining a viable, cost-competitive product. As volumes of current generation chips produced by a typical manufacturing plant approach several million per month, slicing fractions of a second off per unit, manufacturing times can increase total throughput significantly. These high volumes also mean that a small percentage increase in product yield greatly increases the number of functioning units. In an ideal situation, two or more production steps can be accomplished in parallel.
One step of semiconductor manufacture that is not without problems is the die-leadframe attachment. During the manufacturing process, the die is attached to the leadframe, wires are connected from the bond pads on the die to the "fingers" on the leadframe, then the die is encapsulated in a protective plastic casing. The leadframe, part of which will eventually form the conductive leads of the component, contains an area to which the die is attached, called the "paddle." The die can be glued to the paddle, or attached with a tape. FIG. 1 shows a leadframe with a die wirebonded to it.
Below is a partial list of current methods of attaching the die to the leadframe:
1. Epoxy Paste--The epoxy is dispensed on the die paddle area of the leadframe. The die is placed on the uncured epoxy and held by a surface contact tool or an edge contact only tool (collet). The die is lowered into the epoxy by a surface contact tool or an edge contact only tool (collet). The die is pressed down into the epoxy by the tool and held in place long enough to ensure adhesion. X-Y movement (scrub) is sometimes used to increase adhesion and speed the process. This process requires a follow-on cure in a separate cure oven.
2. Epoxy Film--An epoxy film is dispensed on the die paddle of the leadframe and the die is lowered down to the film surface. Bonding is accomplished with pressure. This process requires a follow-on cure in a separate cure oven.
3. Epoxy Film on Tape--An epoxy film that is applied to both sides of a supporting tape is dispensed on the die paddle. Pressure is applied to the die to improve bonding, then the assembly is cured in a separate curing step.
4. Eutectic--A metal with a low melting temperature (solder) is dispensed onto the leadframe paddle. A die is placed on the dispensed metal. Adhesion is obtained by an intermixing of the die backside and the metal. Controlled pressure, scrub, and temperature are used. No follow-on cure is required.
5. Soft Solder--Same process as in Eutectic except that the metal does not mix with the backside material.
6. Glue--A conductive or non-conductive glue can be used as required. The glue would normally be quick set with no follow-on cure required.
Various problems are associated with the connection of the die to the die paddle, and with the connection of the wires from the die pad to the lead fingers. A few of the difficulties associated with the die and leadframe attachments are described below.
A. Corner crack--Occasionally a corner of the die will break, thereby making the semiconductor useless. This can result from an uneven coefficient of expansion between the die and the adhesive used to secure the die to the die paddle. After the die is attached to the leadframe, the assembly is heated at the wire bond step to attach the wire to the die pad. If the die and the glue expand at different rates, the corner of the die may crack. Corner crack can also occur from stress on the die due to shrinkage of the glue as it cures, although in recent years chemical improvements in glue has reduced this cause of corner crack.
B. Lead movement--Lead movement occurs after wire bonding. The lead fingers are relatively long for their thickness, and therefore can bend and move around quite easily. As the assembly is transported to location of the encapsulation step, the wire connections are often broken.
C. Die Shrink--One major objective of semiconductor manufacturing is to make the surface area of the die as small as possible in order to maximize the number of die per wafer. Meeting this objective results in a higher yield, more product for the same quantity of materials, and a lower price to customers. One problem resulting from shrinking the die is that a smaller leadframe must be produced to provide for a reliable length of bond wires. Wires that are over 0.100" long can create reliability problems, such as shorts to the silicon and between the bond wires.
The changeover to a new leadframe layout requires a redesign of the leadframe and purging of any extra leadframe stock. The changeover can incur great capital expense if the leadframe is stamped, and tooling costs can be high.
D. Paddle--The paddle of the leadframe itself is stamped to a lower plane during the manufacturing process, thereby positioning the bottom of the die below the fingers on the leadframe. See FIG. 2. The paddle downset allows for a thinner packaged semiconductor than if the die paddle is not downset, the reduction in package thickness being equal to the thickness of the leadframe. Having a paddle downset also creates problems, as a leadframe with a paddle downset is not as manufacturable as a leadframe without a paddle downset. The paddle downset requires specialized fixtures which are not necessary for leadframes without the downset.
The paddle itself also creates extra expense. The die paddle is typically gold or silver plated along with the conductive leads, in part because the paddle cannot economically be masked during plating of the conductive leads. The leads are plated to improve their conductive properties, but the plating of the paddle serves no functional purpose. This unnecessary plating of the paddle with gold or silver adds unnecessary cost to the product.
A method of attaching the die to the leadframe which solves many of the above problems is to replace the die paddle with a tape comprising a nonconductive plastic or polyimide carrier material with an adhesive on one or both sides. Several uses of the tape are possible. In one use, referred to as "chip on leadframe," an adhesive is applied to both sides of the tape which is sandwiched between the leadframe on the bottom and the die on the top, as shown in FIG. 4. Another use of the tape, "chip on tape," stacks the leadframe and the die on top of the tape, with each being adhered to the same side of the tape as shown in FIG. 5.
Using the tape, the chances of corner crack are reduced as the polyimide tape will give somewhat as the adhesive is curing. Lead movement is also prevented with the tape, as the tape holds the leads in place while the assembly is being transported to encapsulation. Also, the problems associated with the paddle downset are prevented as the tape does not require the leadframe to have a die paddle at all, as can be seen in FIG. 3. Finally, the tape allows the same leadframe design to be used in cases where the die is shrunk. Normally, the die paddle is manufactured about 20 mils larger than the size of the die in both length and width to allow for adhesive which might flow from between the die and the paddle during die attach. With the tape, there is no overflow and therefore the leadframe can be manufactured to a closer tolerance from the start. When the die shrinks, the original leadframe design can be used without die bond wires exceeding the 0.100" maximum length.
Despite its advantages, the polyimide tape as used to secure the die to the leadframe requires many steps to make it work. In the chip on leadframe implementation, adhesive is placed on one side of the tape, the tape is placed on the lead fingers, adhesive is spread on the other side of the tape, the die is placed on the tape, and the adhesive is allowed to cure. This requires more steps than die attachment using a die paddle, as with the die paddle implementation, adhesive is spread on the die paddle, the die is placed in the adhesive, and the adhesive is allowed to cure.
It is an intent of this invention to reduce the number of steps required in the die attach process in order to increase the overall quality and quantity of the product and to reduce time and cost of manufacture.